The present invention relates to a device and a method for the determination of surface properties.
The invention is described with reference to painted surfaces, in particular to the surfaces of motor-vehicle bodywork. It is, however, pointed out that the present invention may also be used in the case of other surfaces.
In the case of numerous technical products, the quality of the visible surface is a crucial feature of the overall impression of the product.
In general, the visual impression of surfaces is decisively influenced by their quality, e.g. their colour, evenness, gloss, orange peel, Distinctness Of Image (DOI), etc. It is, in particular, the evenness or unevenness of the surface that here plays a critical role in respect of its visual impression.
Motor vehicles are usually provided with high-gloss paintwork, the gloss rating of which is generally vastly superior to the gloss rating of other surfaces, e.g. furniture surfaces and the like. The high gloss of the paints used and the relatively large surfaces demand extremely careful preparation of the surfaces to be coated, and very careful application of a base coat. Known from the prior art are devices that, in order to identify quality defects in painted bodywork surfaces, enable an objective characterisation or evaluation of surfaces by analysis of light radiated onto them. These devices make it possible e.g. to detect and quantitatively evaluate unevennesses in the paint layers.
Any unevenness of surfaces is determined by many different factors, such as the mean deviation from an ideally smooth surface, periodicities of individual deviations, etc. In the industrial manufacture of motor-vehicle surfaces or bodywork, a requirement exists to characterise or classify unambiguously any unevenness occurring, and/or the visual impression brought about by such unevenness.
The paints normally used in the automotive industry exhibit not just an external paint layer, but also base coats located beneath this external paint layer. In order to produce final layers of satisfactory quality, it is necessary to check the quality of the relevant base coats too. However, the problem then occurs that these base coats reflect light radiated onto them relatively weakly, and any evaluation of this base coat is therefore extremely complex. Known from DE 10 2004 037 040 A1 is a device in which a light-radiating apparatus radiates light beams in infrared-range wavelengths onto the surface. The use of infrared light ensures that sufficient light is reflected to enable evaluation to take place.
Using this apparatus, conclusions about unevennesses in the base coat are drawn by means of the reflected light. In other words, the base coat is evaluated not directly but indirectly using the reflected light or the directions of the reflected light. With a multiplicity of base coatings, however, even the use of infrared light does not enable evaluation, since even this is reflected too weakly.
Also known from the prior art are roughness measuring devices, which mechanically scan a surface and emit characteristic values for its roughness. These devices are, however, stationary and extremely cumbersome to operate, making them unusable for the examination of e.g. paints.
It would therefore be desirable to create a system and a device for characterising the visual impression of surfaces, which device may be used, in particular, on weakly reflective surfaces. This system is to be simple to operate and portable, in order that it can be used for diverse applications.